1. Field of the Invention
This invention relates to a rudder design for ships traveling at high speeds for reduction of cavitation by using a twisted and balanced rudder (100). The rudder (100) is associated with a propeller (115) arranged on a drivable propeller axis (PA), where the rudder (100) is connected to a rudder post (140).
2. Description of the Related Art
Ships' rudders, such as balanced rudders or balanced profile rudders—with or without a linked fin, are known in various embodiments. Also known are ships' rudders having a twisted rudder blade consisting of two superposed rudder blade sections, whose nose strips facing the propeller are laterally offset in such a manner that one nose strip is offset to port and the other nose strip is offset to starboard.
Thus, JP(A) Sho 58-30896 describes a rudder for ships having a twisted rudder blade consisting of an upper and a lower part, wherein both parts are twisted in their directions facing the propeller. More specifically, the rudder is twisted in such a manner that only the regions of the two parts relating to the nose strips are laterally offset. Whereas the regions extending to the end strips of the two parts have the same cross-sectional profiles and the same cross-sectional dimensions.
GB 332,082 likewise discloses a ships' rudder having a twisted rudder blade whose profile regions facing the propeller, namely the nose strips to starboard and port, are laterally flared and where the nose strips are configured to taper to a tip. The cross-sectional profiles of the two rudder blade sections are configured so that the side wall surfaces of the two rudder blade sections located on the port and starboard side run free from curvature and rectilinearly between the end strips to each laterally bent nose strips so that the side wall surfaces have no outwardly curved regions having different radii of curvature. In addition, the profile configuration of the rudder blade is such that the two cross-sectional surfaces of the two rudder blade surfaces located one above the other are the same size and extend over the entire height of the rudder blade. Due to the nose strips tapering to a peak, sharp-edged indentations are formed, which are exposed to cavitation and destruction. An improvement in the propulsion is achieved with the new rudder configuration disclosed in this specification.
The speeds of modern ships are continually increasing. As a result of the fast flowing water velocities associated with the higher speeds of modern ships, the loading on the propeller and on the rudder is increasing. The symmetry of the profile of previous rudder blade designs creates under-pressure zones on the rudder surface which then leads to cavitation and eventual erosion. Cavitation takes place on those points of the rudder blade at which the flow is extremely accelerated. In this case, the strong rotational water flow of the propeller impacts on the rudder blade surface at high speed. As a result of this strong acceleration, the static pressure drops below the vapor pressure of the water, resulting in the formation of vapor bubbles which abruptly implode. These implosions lead to destruction of the rudder blade surface. This results in expensive repairs and frequently new rudder blades must be used.